RE: [PATCH] arm64: mm: decrease the section size to reduce the memory reserved for the page map

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: Mike Rapoport [mailto:rppt@xxxxxxxxxxxxx]
> Sent: Saturday, December 5, 2020 12:44 AM
> To: Will Deacon <will@xxxxxxxxxx>
> Cc: liwei (CM) <liwei213@xxxxxxxxxx>; catalin.marinas@xxxxxxx; fengbaopeng
> <fengbaopeng2@xxxxxxxxxxxxx>; nsaenzjulienne@xxxxxxx; steve.capper@xxxxxxx;
> Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>;
> linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; butao
> <butao@xxxxxxxxxxxxx>
> Subject: Re: [PATCH] arm64: mm: decrease the section size to reduce the memory
> reserved for the page map
> 
> On Fri, Dec 04, 2020 at 11:13:47AM +0000, Will Deacon wrote:
> > On Fri, Dec 04, 2020 at 09:44:43AM +0800, Wei Li wrote:
> > > For the memory hole, sparse memory model that define SPARSEMEM_VMEMMAP
> > > do not free the reserved memory for the page map, decrease the section
> > > size can reduce the waste of reserved memory.
> > >
> > > Signed-off-by: Wei Li <liwei213@xxxxxxxxxx>
> > > Signed-off-by: Baopeng Feng <fengbaopeng2@xxxxxxxxxxxxx>
> > > Signed-off-by: Xia Qing <saberlily.xia@xxxxxxxxxxxxx>
> > > ---
> > >  arch/arm64/include/asm/sparsemem.h | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/arch/arm64/include/asm/sparsemem.h
> b/arch/arm64/include/asm/sparsemem.h
> > > index 1f43fcc79738..8963bd3def28 100644
> > > --- a/arch/arm64/include/asm/sparsemem.h
> > > +++ b/arch/arm64/include/asm/sparsemem.h
> > > @@ -7,7 +7,7 @@
> > >
> > >  #ifdef CONFIG_SPARSEMEM
> > >  #define MAX_PHYSMEM_BITS	CONFIG_ARM64_PA_BITS
> > > -#define SECTION_SIZE_BITS	30
> > > +#define SECTION_SIZE_BITS	27
> >
> > We chose '30' to avoid running out of bits in the page flags. What changed?
> 
> I think that for 64-bit there are still plenty of free bits. I didn't
> check now, but when I played with SPARSEMEM on m68k there were 8 bits
> for section out of 32.
> 
> > With this patch, I can trigger:
> >
> > ./include/linux/mmzone.h:1170:2: error: Allocator MAX_ORDER exceeds
> SECTION_SIZE
> > #error Allocator MAX_ORDER exceeds SECTION_SIZE
> >
> > if I bump up NR_CPUS and NODES_SHIFT.
> 
> I don't think it's related to NR_CPUS and NODES_SHIFT.
> This seems rather 64K pages that cause this.
> 
> Not that is shouldn't be addressed.

Right now, only 4K PAGES will define ARM64_SWAPPER_USES_SECTION_MAPS.
Other cases will use vmemmap_populate_basepages().
The original patch should be only addressing the issue in 4K pages:
https://lore.kernel.org/lkml/20200812010655.96339-1-liwei213@xxxxxxxxxx/

would we do something like the below?
#ifdef CONFIG_ARM64_4K_PAGE
#define SECTION_SIZE_BITS	27
#else
#define SECTION_SIZE_BITS	30
#endif

> 
> > Will
> 
> --
> Sincerely yours,
> Mike.

Thanks
Barry